The present invention relates to a method and apparatus for phase and amplitude modulation of a baseband signal and a system incorporating the same.
In particular, the present invention relates to a method and apparatus for phase and amplitude modulation of a digital signal and a system incorporating the same.
Digital communication equipment is used in a wide variety of devices for the transmission of digital information. Such information includes numerical data in computers and digital encodings of voice in telecommunications systems.
The majority of modulation formats involve impressing a time varying amplitude and phase characteristic onto a carrier wave in a predetermined manner whereby there is a fixed relationship between the information that the signal is to carry and the instantaneous values of the phase and amplitude. For a carrier as defined in equation (1):
Carrier=Re[e jxcfx89t]xe2x80x83xe2x80x83(1)
and the modulation defined by equation (2):
Modulation=Re[r(t) e jxcfx86(t)]xe2x80x83xe2x80x83(2)
then the modulated signal is defined equally by equations (3) and (4):
Modulated Signal=Re [r(t) e j(xcfx89t+xcfx86(t)]xe2x80x83xe2x80x83(3)
Modulated Signal=r(t) cos(xcfx89t+xcfx86(t))xe2x80x83xe2x80x83(4)
Many of the commonly used modulation formats involve moderate or large amounts of amplitude variation in the signal envelope and require highly linear power amplifiers to maintain signal distortion and spectral regrowth at acceptably low levels. As a consequence, power amplifiers, for example in wireless telephone handsets, tend to exhibit poor power-added efficiency with a resultant high DC power consumption and short battery life for the handsets.
Known partial solutions for alleviating these problems use either amplifier linearization techniques or constant envelope modulation formats. Such amplifier linearization techniques are highly complex and thus suitable for use only in base stations. They are generally too complex and expensive for use in handsets. Additionally, constant envelope modulation formats, such as that employed in GSM systems, permit encoding of only 1 bit per symbol rather than the 2 bits per symbol of, for example, xcfx80/4 DQPSK.
The present invention seeks to provide an improved method and apparatus for simultaneously phase- and amplitude-modulating a radio frequency (RF) or microwave carrier.
According to a first aspect of the present invention there is provided a baseband signal modulator comprising first and second phase modulators arranged to receive a common reference signal and each arranged to receive a phase-encoded data signal and to provide a phase-encoded carrier signal derived from said data signal and said common reference signal, circuit means arranged to sum said phase-encoded carrier signals, whereby to provide a phase and amplitude modulated carrier signal.
Preferably, each said phase-encoded carrier signal is constant-envelope.
Preferably, both phase and amplitude of said phase and amplitude modulated carrier signal may be varied dependent upon the phase of each said input data signal.
Preferably, a phase modulator comprises a phase-locked loop and an amplitude-adjustment circuit.
The phase-locked loop may be an offset phase-locked loop.
The amplitude-adjustment circuit may comprise a coarse attenuator and a power amplifier.
Advantageously, the power amplifiers can be run much closer to their compression point than is the case in known modulators, and the power amplifiers can be of a less linear and more power-efficient design.
Advantageously, the more power-efficient design leads to longer battery life where the present invention is employed in mobile handsets.
Advantageously, spectral re-growth and consequential interference into the adjacent channels is much reduced because the amplifiers run with a constant power envelope.
Advantageously, since two amplifiers are used, each one only carries half the required power thereby reducing the cost of the output devices. Although two amplifiers are required this may still represent a significant cost reduction for a base station.
Advantageously, all the advantages of loop filtering by the method of phase modulation in the loop, as currently exploited in GSM systems, can be gained for non-constant envelope modulation formats.
Advantageously, the present invention may use components already developed and available for existing GSM systems, thereby both minimizing chip re-design costs and keeping manufacture costs low.
Advantageously, since amplitude control is inherent in the technique, the control of final power output level can be incorporated into the phase modulation of the two signal arms.
Preferably, input data signals are phase-encoded using distinct phases.
The signal modulator may additionally comprise a signal source providing said reference signal.
Each modulator may be arranged to receive a phase-encoded digital data signal.
Preferably, the signal modulator additionally comprises digital signal processing means whereby to provide said phase-encoded data signals.
According to a further aspect of the present invention there is provided a telecommunications system comprising one or more signal modulators according to claim 1.
In a preferred embodiment, said phase and amplitude modulated carrier signal is a radio frequency signal.
According to a further aspect of the present invention there is provided a wireless transmission system comprising a signal modulator according to the present invention and an antenna and a low-pass filter arranged to receive said phase and amplitude modulated carrier signal and to provide it to said antenna.
Use of GSM components also enables use of simple, low-cost output filters.
According to a further aspect of the present invention there is provided a wireless telecommunications system comprising one or more signal modulators according to claim 1.
According to a further aspect of the present invention there is provided a method of generating a phase and amplitude modulated signal comprising the steps of: providing a reference input signal; providing first and second data input signals; generating a phase-modulated carrier signal from each said data signal and the said reference signal; combining said phase-modulated carrier signals to provide a phase and amplitude modulated carrier signal.
According to a further aspect of the present invention there is provided a method of generating a phase and amplitude modulated signal comprising the steps of: providing target signal amplitude and phase values; providing a reference input signal; providing first and second data input signals characterised by said target signal amplitude and phase values; phase-modulating each said data signal separately onto said reference signal; combining said phase-modulated signals to form a phase and amplitude modulated signal characterised by said target signal amplitude and phase values.
Advantageously, the modulation format may be wholly controlled by the baseband DSP sections of the radio and, where the PLL""s can be tuned to the carrier frequencies, the present invention lends itself to use in multimode type radios.
Advantageously, more information can be sent through a given channel if both phase and amplitude modulation are employed.
Advantageously, it is easy to change output modulation schemes by changing the phase values of each DSP output signal.
Advantageously, the present invention does not require highly-linear power amplifier circuits, thereby reducing manufacturing costs.
The invention is also directed to a method by which the described apparatus operates and includes method steps for carrying out every function of the apparatus.
The invention also provides for a system for the purposes of digital signal processing which comprises one or more instances of apparatus embodying the present invention, together with other additional apparatus.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.